Process of forming metal coatings on metal strip by electrophoretic deposition



July 18, 1967 A. E. JACKSON 3,331,762 7 PROCESS OF FORMING METALCOATINGS ON METAL STRIP BY ELECTROPHORETIC DEPOSITION Filed April 18,1963 INVENTOR A. E. Jackson .ATIORNEYS United States Patent PROCESS 6FFORMING METAL COATINGS 0N METAL STRIP BY ELECTROPHORETIC DEPOSITIONAlbert Edward Jackson, Swansea, Wales, assignor to The British Iron andSteel Research Association Filed Apr. 18, 1963, Ser. No. 273,969 2Claims. (Cl. 204-181) This invention relates to the formation of metalcoatings on metal strip by electrophoretic deposition.

In British Patent No. 884,797 to Salt and Lewis there is described andclaimed a process for the formation of metal coatings on, inter alia,metal strip which comprises electrophoretically depositing the coatingmetal on to the strip from a suspension of finely divided coating metal,drying the coated strip to remove any electrophesis medium removed fromthe electrophoresis bath, rolling the coated strip to compact thecoating, and then heating the coated strip to obtain a tightly bondedcoating.

As the coated substrate is removed from the electrophoresis bath, thecoating is in a non-coherent and nonadherent form and if the coatedsubstrate, in this condition, is passed over a roll in order to changeits direction of travel, i.e. so as to pass it to the drying station, apart of the coating becomes transferred to the roll with consequentimpairment of the final coating after the rolling and heatingtreatments.

According to a first aspect of the present invention, therefore, thecoated strip is passed in a straight pass from the electrophoresis baththrough the drying station to the compacting rollsand the coated stripis not contacted by any other roll until it reaches the compactingrolls. Once the coated strip has been compacted there is substantiallyno tendency for the coatingto adhere to rolls and the coated strip maybe passed around a roll in order to change its direction of travel.

The compacting pressure applied by the compacting rolls is preferablyfrom 1 /2 to 10 tons per inch width of the coated strip, the optimumpressure for any particular coated strip within this range beingdetermined by the hardness of the strip and the diameter of thecompacting rolls.

In carrying out this procedure it is preferred to pass the stripvertically upwards through the electrophoresis cell and the dryingstation to the compacting rolls. In some cases it may not be convenientto locate a rolling mill capable of applying pressures within the aboverange, at the required distance above ground level, that is at a heightsuch that the electrophoresis bath and the drying station can beaccommodated vertically below the rolling mill, and in these cases arolling mill capable of applying a relatively low pressure can be usedinstead. Such a rolling mill can be used, as we have found thatcompacting pressures below those required to ensure adequate adhesion ofthe coating in the final product, are sufficient to prevent the coatingadhering to rolls. Thus while it is preferred to use compactingpressures of from 1 /2 to 10 tons per inch width in order to developgood adhesion of the coating in the final product, we have found that apreliminary compaction with a pressure of from /2 to 1 ton per inchWidth (which, it will be apparent, requires a much lighter and smallerrolling mill than that required for full compaction) substantiallyremoves the tendency of the coating to adhere to rolls. When preliminarycompaction is used, a light rolling mill capable of exerting a pressureof from /2 to 1 ton per inch width is located vertically above theelectrophoresis bath and drying station, and the coated strip is passedfrom this rolling mill or from a roll located above this rolling mill tothe rolling mill for effecting full compaction, which can be located atground level.

There is a tendency for coating metal to adhere to the rolls whicheffect the first compaction of the coating (whether it be a preliminaryor a full compaction) and in order to prevent coating metal being builtup on the rolls in contact with the metal coating and the consequentialdamage to the coating such deposits would cause, it is preferred toprovide the rolling mill with one or more pads of, or rolls coveredwith, a soft fibrous material, such as lambswool or felt, and with oneor more doctor knives, a pad or roll and a knife bearing against eachroll in contact with the coating so as to clean its surface. We havefound that a more eflicient cleaning action is obtained with a softfibrous material, such as lambswool or felt, than with a brush.

The drying effected by the drying station referred to above issubstantially complete and since the amount of electrophoresis mediumremoved from the electrophoresis cell is quite significant and increaseswith the strip speed, it is preferred to associate the drying stationwith a solvent recovery plant wherein the organic solvent carried out ofthe electrophoresis cell by the coated strip and volatilised therefromin the drying station, is condensed and recycled to the electrophoresiscell. Such a solvent recovery plant renders the electrophoretic coatingprocess more economic, but its operating costs must, of course, formpart of the overall operating costs of the process.

We have found that the operating costs of the process can be reduced byreducing the amount of drag-out, i.e. the amount of electrophoresismedium carried out of the cell by the coated strip, and that drag-outcan be substantially and effectively reduced by the use of air knives,that is jets of air having a component of velocity directly opposite tothe direction of travel of the strip and impinging at an acute angle onthe strip across its width. The effect of the air knives is to blow theentrained electrophoresis medium back towards the electrophoresis celland in order that the minimum amount of electrophoresis medium should belost, it is preferred, again, that the coated strip should be passedvertically upwards from the electrophoresis cell so that entrainedmedium blown off the coated strip by the air knives can fall back intothe cell. The reduction in the amount of drag-out by use of air knives,also reduced the heating required to dry the coated strip.

In order to indicate the effectiveness of air knives, comparative runswere made at different rates of withdrawal from the electrophoresiscell. In all cases the substrate was steel strip, the coating metalaluminium powder, the electrophoresis medium 20% aqueous methylatedspirit containing 0.5 millimole/litre of aluminium nitrate, the currentdensity 10 amps/sq. ft. and the coating thickness (after compaction)0.001 inch. The air knives were airheaders tapering to a slit 3 mm. wideand extending across the width of the strip which produced jetsimpinging at an angle of 30 on to the strip.

Drag-out (Gallons/1,000 sqjt.) Velocity of With- Reduction, drawal,ftjmin. Percent Without With air knife air knife We have found that whenthe liquid content of the coating has been reduced, by air knives and/orby heating, to between 20% and 75% by weight, based on the weight of thecoating, the coated strip can be contacted with a roll having a surfaceof a suitable non-adherent material 3 for the purpose of changing itsdirection of travel without damage to the coating. That is to say,provided that the coated strip is neither too dry nor too wet, it can bepassed over a roll and directed to a floor mounted rolling mill for thepurpose of effecting compaction of the coating and it is not necessaryto use a mill mounted above the electrophoresis cell.

Suitable non-adherent materials for the roll surface are,

'for example, chromium or nickel plated steel, polyethgrammatically inthe accompanying drawing. A run was carried out with steel strip 25 mm.wide and having a thickness of 0.6 mm. The strip was passed verticallyup- Wards through between two vertical sheet anodes 9, spaced 2 inchesapart, in an electrophoresis cell 10. The cell contained a continuouslycirculated suspension of aluminium' powder in 20% aqueous methylatedspirit containing 0.5 millimole/ litre of aluminium nitrate. The currenton each side of the strip was 2.25 amps and the voltage 75 volts; thealuminium coating thickness was 25 microns. The strip was passedvertically upwards between two air knives 11 and two radiant electricheaters 12 to a chromium plated steel roll 13 having a diameter of 0.2metre. From the'roll, it was passed downwardly, at an acute angle to thehorizontal through a drying chamber 15 continaing a high frequencyelectric drier 14 and through which a current of air indicated by arrows16 was passed, to a floor mounted rolling mill 17 where compaction waseifected. The air knives were mounted 1 inch above the opening of theelectrophoresis cell and directed jets of air at an angle of 30 on tothe coated strip and the radiant heaters were mounted 2 feet above theair knives at a distance of 2 inches from the strip. The line speed was3 metres/minute and the fall on the strip (that required to pull thestrip over the roll) was 45 kilograms.

Under these conditions, a uniform and well adherent aluminium coatingwas obtained and there was no damage to the coating as a result of itscontact with the roll. The same result was obtained in another run underthe same conditions but using strip 0.25 mm. thick.

As indicated above electrophoresis is preferably effected while movingthe strip vertically upwards through the electrophoresis bath. Thelatter is advantageously contained in a vertically elongated tank, thebottom of which is provided with a stufling box through which the stripis passed into the tank. We have found that although there is a tendencyfor the electrophoresis medium to escape through the stufilng' box inthe first few minutes of operation, particles of coating metal rapidlyfill up any gaps and interstices so that an effective seal is formed.

Whilst various methods are-available formaintaining the coating metalparticles in suspension, it is preferred to do this by circulating thesuspension in a closed circuit which includes the electrophoresis tank,the suspension being introduced at the bottom of the tank andbeing takenofr" at the top. Advantageously the cross-section of the tank is keptsmall, ie so that it is not much larger than is required to accommodatethe strip and the anode or anodes, and the suspension is introducedthrough two inlets located at the base of opposite walls of the tank.Removal of the suspension from the top of the tank is preferablyeffected by using a tank which is open at the top and allowing thesuspension to flow over the whole of the periphery of the top of thetank to be collected in a header located some way below the top of thetank, the suspension being discharged from the header to the returncircuit.

With this vertical arrangement of strip and anode or anodes, it ispreferred that the latter should not extend to the surface of the bath,but should be some 2 or 3 inches short of the surface, in order toreduce the occurrence of surface defects in the coating due to drag-outfrom the bath. We have found that the surface quality of the coatingobtained is improved by circulating the suspension at a faster rate thanthe minimum required to maintain the coating metal particles insuspension.

After full compaction of the coated strip it'is subjected to heattreatment in order to develop maximum adhesion of the coating to thesubstrate and such heating is prefer ably effected with the coated stripin coil. Undue bending 'of the coated strip before the heat treatmenttends to cause the coating to delaminate from the substrate and it istherefore desirable to coil the strip on a mandrel which is sufficientlylarge to avoid any undue bending taking place. The amount of bendingwhich can be tolerated is indirectly related to the substrate thickness;thus, for example, it is preferred to use a mandrel having a diameter ofnot less than 12 inches when the substrate is 0.012 in. thick,

and having a diameter of not less than 16 inches when the substrate is0.022 in. thick.

I claim:

1. In a process for the formation of metal coatings on metal strip byelectrophoretically depositing the coating metal on to the strip from asuspension of finely divided coating metal, drying the coated strip toremove entrained electrophoresis medium therefrom, rolling the coatedstrip to compact the coating, and then heating the coated strip toobtain a tightly bonded coating, the improvement which comprisessubjecting the coated strip onleaving the elec;- trophoresis cell to theaction of air knives which direct jets of air having a component ofvelocity directly opposite to the direction of travel of the strip andimpinging at an acute angle on the strip across its width.

2. A process according to claim 1, in 'which the jets of air impinge atan angle of 30 on both sides of the strip'.

References Cited UNITED STATES PATENTS 1,897,902 2/1933 Harsanyi 204 1s13,034,971 5/1962 Seaward 204 1s1 3,091,547 5/1963 Jones 117 -65.2

3,096,567 7/1963 Ross et al 117-652 FOREIGN PATENTS 884,797 12/1961GreatBritain.

JOHN H. MACK, Primary Examiner. E. ZAGARELLA, Assistant Examiner,

1. IN A PROCESS FOR THE FORMATION OF METAL COATINGS ON METAL STRIP BYELECTROPHORECALLY DEPOSITING THE COATING METAL ON TO THE STRIP FROM ASUSPENSION OF FINELY DIVIDED COATING METAL, DRYING THE COATED STRIP TOREMOVE EXTRAINED ELECTROPHORESIS MEDIUM THEREFROM, ROLLING THE COATEDSTRIP TO COMPACT THE COATING, AND THEN HEATING THE COATED STRIP TOOBTAIN A TIGHTLY BONDED COATING THE IMPROVEMENT WHICH COMPRISESSUBJECTING THE COATED STRIP ON LEAVING THE ELECTROPHORESIS CELL TO THEACTION OF AIR KNIVES WHICH DIRECT JETS OF AIR HAVING A COMPONENT OFVELOCITY DIRECTLY OPOSITE TO THE DIRECTION OF TRAVEL OF THE STRIP ANDIMPINGING AT AN ACUTE ANGLE ON THE STRIP ACROSS ITS WIDTH.